PloneBatch: define __bool__ as copy of __nonzero__.

[mauritsvanrees]

Python 2 calls __nonzero__, Python 3 calls __bool__.
Since __bool__ was not defined, Python 3 called len(batch) instead, leading to these warnings when a template has code like tal:condition="batch":

Products/PageTemplates/Expressions.py:253: DeprecationWarning: Using len() is deprecated.
Use the `length` attribute for the size of the current page, which is what we return now.
Use the `sequence_length` attribute for the size of the entire sequence. 
  return bool(value)

(I now think we should undo this deprecation, but that is something for a different issue.)

[mauritsvanrees]

@jenkins-plone-org please run jobs